Small Aquarium Research Articles

Signaling against the Wind: Modifying Motion-Signal Structure in Response to Increased Noise

Animal signals are optimized for particular signaling environments [1-3]. While signaling, senders often choose favorable conditions that ensure reliable detection and transmission [4-8], suggesting that they are sensitive to changes in signal efficacy. Recent evidence has also shown that animals will increase the amplitude or intensity of their acoustic signals at times of increased environmental noise [9-11]. The nature of these adjustments provides important insights into sensory processing. However, only a single piece of correlative evidence for signals defined by movement suggests that visual-signal design depends on ambient motion noise [12]. Here we show experimentally for the first time that animals communicating with movement will adjust their displays when environmental motion noise increases. Surprisingly, under sustained wind conditions, the Australian lizard Amphibolurus muricatus changed the structure and increased the duration of its introductory tail flicking, rather than increasing signaling speed. The way these lizards restructure the alerting component of their movement-based aggressive display in the presence of increased motion noise highlights the challenge we face in understanding motion-detection mechanisms under natural operating conditions.

Do cleaning organisms reduce the stress response of client reef fish?

BackgroundMarine cleaning interactions in which cleaner fish or shrimps remove parasites from visiting 'client' reef fish are a textbook example of mutualism. However, there is yet no conclusive evidence that cleaning organisms significantly improve the health of their clients. We tested the stress response of wild caught individuals of two client species, Chromis dimidiata and Pseudanthias squamipinnis, that had either access to a cleaner wrasse Labroides dimidiatus, or to cleaner shrimps Stenopus hispidus and Periclimenes longicarpus, or no access to cleaning organisms.ResultsFor both client species, we found an association between the presence of cleaner organisms and a reduction in the short term stress response of client fish to capture, transport and one hour confinement in small aquaria, as measured with cortisol levels.ConclusionIt is conceivable that individuals who are more easily stressed than others pay a fitness cost in the long run. Thus, our data suggest that marine cleaning mutualisms are indeed mutualistic. More generally, measures of stress responses or basal levels may provide a useful tool to assess the impact of interspecific interactions on the partner species.

Effect of temperature on emergence, survival and infectivity of cercariae of the marine trematode Renicola roscovita (Digenea: Renicolidae)

Marine bivalves harbour a diversity of trematode parasites affecting population and community dynamics of their hosts. Although ecologically and economically important, factors influencing transmission between first (snail) and second (bivalve) intermediate hosts have rarely been studied in marine systems. In laboratory experiments, the effect of temperature (10, 15, 20, 25 degrees C) was investigated on (1) emergence from snails, (2) survival outside hosts and (3) infectivity in second intermediate hosts of cercariae of the trematode Renicola roscovita (Digenea: Renicolidae), a major parasite in North Sea bivalves. Emergence of cercariae peaked at 20 degrees C (2609 +/- 478 cercariae snail(-1) 120 h(-1)) and was considerably lower at 10 degrees C (80 +/- 79), 15 degrees C (747 +/- 384) and 25 degrees C (1141 +/- 334). Survival time decreased with increasing temperature, resulting in 50% mortality of the cercariae after 32.8 +/- 0.6 h (10 degrees C), 26.8 +/- 0.8 h (15 degrees C), 20.2 +/- 0.5 h (20 degrees C) and 16.6 +/- 0.3 h (25 degrees C ). Infectivity of R. roscovita cercariae in cockles Cerastoderma edule increased with increasing temperature and was highest at 25 degrees C (42.6 +/- 3.9%). However, mesocosm experiments with infected snails and cockle hosts in small aquaria, integrating cercarial emergence, survival and infectivity, showed highest infection of cockles at 20 degrees C (415 +/- 115 metacercariae host(-1)), indicating 20 degrees C to be the optimum temperature for transmission of this species. A field experiment showed metacercariae of R. roscovita to appear in C. edule with rising water temperature in April; highest infection rates were in August, when the water temperature reached 20 degrees C. Since another trematode species (Himasthla elongata; Digenea: Echinostomatidae) occurring at the experimental site showed a similar temporal pattern, trematode transmission to second intermediate bivalve hosts may peak during especially warm (> or = 20 degrees C) summers in the variable climate regime of the North Sea.

Salmon Samaritans

Salmon Samaritans

New York Shelf Life: Dirk Westphal’s Acts of Preservation

This article considers four photographic series by New York artist Dirk Westphal. In each of these series, Westphal takes an ephemeral object (telephone booths, snack-cakes, mouthwash and small aquarium fish) and preserves it through a multimedia, multi-step process delivered via photography. His images and the labor to produce them trace and remake the intersection between the two essential components of capitalism: mass production and mass consumption. Yet he reinvents the labor in the form of the artist, the consumer in the form of the viewer. He learns from and transforms the market, collecting and remaking knowledge of urban space for his viewer-consumers.

Resolving mechanisms of toxicity while pursuing ecotoxicological relevance?

In this age of modern biology, aquatic toxicological research has pursued mechanisms of action of toxicants. This has provided potential tools for ecotoxicologic investigations. However, problems of biocomplexity and issues at higher levels of biological organization remain a challenge. In the 1980s and 1990s and continuing to a lesser extent today, organisms residing in highly contaminated field sites or exposed in the laboratory to calibrated concentrations of individual compounds were carefully analyzed for their responses to priority pollutants. Correlation of biochemical and structural analyses in cultured cells and tissues, as well as the in vivo exposures led to the production and application of biomarkers of exposure and effect and to our awareness of genotoxicity and its chronic manifestations, such as neoplasms, in wild fishes. To gain acceptance of these findings in the greater environmental toxicology community, “validation of the model” versus other, better-established often rodent models, was necessary and became a major focus. Resultant biomarkers were applied to heavily contaminated and reference field sites as part of effects assessment and with investigations following large-scale disasters such as oil spills or industrial accidents. Over the past 15 years, in the laboratory, small aquarium fish models such as medaka ( Oryzias latipes), zebrafish ( Danio rerio), platyfish (Xiphophorus species), fathead minnow ( Pimephales promelas), and sheepshead minnow ( Cyprinodon variegatus) were increasingly used establishing mechanisms of toxicants. Today, the same organisms provide reliable information at higher levels of biological organization relevant to ecotoxicology. We review studies resolving mechanisms of toxicity and discuss ways to address biocomplexity, mixtures of contaminants, and the need to relate individual level responses to populations and communities.

Husbandry and medicine of aquatic reptiles

Any pet store in North America has a plethora of reptiles for sale. Most of these are terrestrial. However, there are a few available that must be maintained in aquatic or semiaquatic environments. Some species such as crocodilians and the larger aquatic chelonians, as well as the pelagic or marine reptiles, should be maintained at zoological parks and aquaria. Pet reptiles are those that can be maintained in glass aquaria in home table top environments. Aquatic reptiles, by the nature of their environment—water—are more difficult to keep in a homoeostatic environment. Maintaining pristine water quality is difficult because of rapid changes in the pH, increases in nitrogen levels, and algae production in the home aquarium. Most of the species do not consume all that they are offered to eat, and that excess food along with the feces produced can rapidly change the water quality. The species of aquatic reptiles most commonly kept in captivity are the following chelonians; red-eared slider ( Trachemys scripta elegans), Eastern spotted turtle ( Clemmys guttata), diamond-backed terrapin ( Malaclemmys terrapin), Blandling’s turtle ( Emydoidea blandingii), painted turtle ( Chrysemys picta), cooter ( Pseudemys floridana), wood turtle ( Clemmys insculpata), and Western pond turtle ( Clemmys marmorata). Rarely seen anymore in a home environment are crocodilians such as caimans and alligators. These species can be aggressive, rapidly outgrow a small aquarium, and, due to the growth rate, be difficult to feed regularly. Due to these factors, crocodilians should only be kept by expert biologists at reputable zoological parks and aquaria.

A computer image processing system for quantification of zebrafish behavior

The tropical zebrafish ( Danio rerio) has frequently been used for investigating developmental biology. Here, we developed a computer image processing system for quantifying zebrafish behavior. We could acquire an image of zebrafish freely moving in an aquarium using a CCD camera through a graphic I/O board. To acquire the image of moving zebrafish in real time, we required high spatial (256×256 pixels) and temporal (10 frames/s) resolution. Such a high speed of data analysis was accomplished using a skipping search method. By using a small aquarium, trackings of newborn zebrafish could be traced. The velocity of adult zebrafish (7.2 cm/s) was far faster than that of newborn zebrafish (1.8 cm/s). Furthermore, by separation of occluded images of two fish, we could acquire images of the two zebrafish. They behaved as in a school in which one fish chased the other. The chasing was defined by the distance, angle and approach of the two fishes. The chasing ratio of pairs of zebrafish was 37%, whereas those of pairs of different fish were significantly reduced to less than 20%. The present image processing system is a very useful tool for quantitatively scoring the schooling behavior of multiple fish.

Aspects of aspect

Performing an action and concurrently describing it creates possibilities for observing how speech and action are synchronized and mutually shape each other. The results to be described show that iconicity is an organizing principle of motion control — actions are performed in relation to speech in such a way as to create an image of the meaning in part. Four English-speaking subjects described their goal-directed actions, step by step, as they assembled a small aquarium. Among the factors observed to have an impact on synchrony are, in speech, perfectivity (imperfective or perfective) and, in action, the logical form of the action (Vendler’s activities, accomplishments and achievements). These factors must be considered jointly. Depending on the logical form, speakers placed the linguistic center (‘L-Center’) of the description — the satellite or a preposition in the case of activities and accomplishments; the verb itself in the case of achievements — in such a way as to recreate the aspectual viewpoint concurrently encoded in speech. Imperfective aspect was indexed with progressive verbs. L-Centers with this aspect were placed inside or prior to the completion of the action. Perfective aspect was indexed with non-progressive verbs. L-Centers of this kind were placed after the action. In addition, the actions themselves were longer with imperfective aspect, even when the same logical form of action was involved. Non-functional movements were added to goal-directed actions to help create the imperfective aspect, and both speech and action timing were adjusted to create the aspectual viewpoint. These patterns are reminiscent of the perfectivity differences in gesture performance described by S. Duncan (2003) for Mandarin and English. Motor movements adapt to language in a way similar to gestures. Theoretically, there is a three-sided synchrony of two forms of motor action (speech, manual manipulation) and the logical form of the action. The theoretical discussion considers two ramifications: the implications for ‘cognitive being’ of building aspect into action, which suggests a mechanism of the mutual shaping of speech and action; and the brain organization that might underlie the three-sided synchrony, with emphasis on the role of BA 44 and BA 45 (‘mirror neurons’), collectively ‘Broca’s Area’, in the orchestration of actions under some meaning (actions of gesture, manipulation, and speech); these areas include language but are broader in function than the classical term ‘language area’ implies.

Zebra fish: an uncharted behavior genetic model.

The zebra fish has been a preferred subject of genetic analysis. It produces a large number of offspring that can be kept in small aquaria, it can be easily mutagenized using chemical mutagens (e.g., ethyl nitrosourea [ENU]), and high-resolution genetic maps exist that aid identification of novel genes. Libraries containing large numbers of mutant fish have been generated, and the genetic mechanisms of the development of zebra fish, whose embryo is transparent, have been extensively studied. Given the extensive homology of its genome with that of other vertebrate species including our own and given the available genetic tools, zebra fish has become a popular model organism. Despite this popularity, however, surprisingly little is known about its behavior. It is argued that behavioral analysis is a powerful tool with which the function of the brain may be studied, and the zebra fish will represent an excellent subject of such analysis. The present paper is a proof of concept study that uses pharmacological manipulation (exposure to alcohol) to show that the zebra fish is amenable to the behavioral genetic analysis of aggression and thus may allow us to reveal molecular mechanisms of this behavioral phenomenon relevant to vertebrates.

Observations on rearing damselflies under laboratory conditions

Abstract Rearing damselflies under laboratory conditions is a promising means of solving a variety of biological questions. Therefore, in order to improve the success of future researchers we felt the need to indicate potential difficulties in carrying out rearing experiments. Laboratory crosses were obtained using virgin animals originating from natural populations in Belgium and Spain. Resulting offspring was maintained, under laboratory conditions, in small aquaria until emergence and in insectaries as adults. Our results show that keeping damselflies during their entire life cycle under artificial conditions can be very difficult. We suggest that future researchers should change water regularly, supply sufficient food, and rear animals at low density or even individually. Furthermore, suggestions are given on type of food, advisable laboratory conditions and female oviposition methodology.

Social interaction and distribution of female zebrafish (Danio rerio) in a large aquarium.

The zebrafish (Danio rerio) is native to freshwater streams and rice paddies in the Ganges River of East India, Bangladesh, and Burma. Although this species is widely used as a genetic model and for developmental research, little is known about its natural behavior (e.g., spatial distribution and social interactions). In the laboratory, zebrafish are usually kept in small aquaria under high densities. The mating behavior of the zebrafish seems to be influenced by the exposure of mating partners to one another several to 24 h before spawning begins at sunrise; female gonadal hormones apparently stimulate males to perform courtship behavior (1, 2). Our preliminary experiments (unpubl. data) showed reproductive suppression of female zebrafish kept together for 4 days before they were separated and mated with a single male overnight. On average, each of these females (n 52) laid only 45 12 (SE) eggs, in comparison to 200 12 (SE) eggs of a female kept isolated for 4 days and then mated with one male (n 15; 150 12 (SE) eggs) or two males (n 30; 180 12 (SE) eggs) overnight. Females kept together in a aquarium behaved aggressively toward each other and developed a dominance hierarchy. These results led us to assume that females would avoid other females if they could. The aim of this study was to investigate spatial and social distribution of female zebrafish in a large aquarium where they could choose between different localities and partners. We observed wild-type zebrafish, aged 10 months, which were kept in 30-l aquaria under a day/night cycle of 14/10 h with light on at 9:00 am before the experiments were started. Four males and four females were selected for each experiment. The eight fish were tagged individually behind the dorsal fin with Floy fish tags, all bearing a different color. These tags did not seem to affect the behavior of the fish. The fish were placed in a large 195-cm 195-cm aquarium filled with 600 l (water height: 16 cm) of formulated freshwater. A grid system of 16 equal squares was drawn on the bottom of the aquarium. In 7 of the 16 squares, artificial plants were placed in order to imitate zebrafish habitat. The fish were left to adjust to their new environment for about 3 h. Each of the five experiments lasted for 5 days. Half-hour observations were made twice daily, with at least 1 h between observation sessions. The observation sessions were split into 1-min intervals (31 intervals per observation), and the location of each fish was noted at every time interval. Newly released fish showed a strong tendency for shoaling, but after about 2–3 h they dispersed to different parts of the aquarium. Thereafter, they spent 99% of their time in one of the seven squares containing artificial plants. We evaluated how often each female was seen either alone or in the presence of only males (1 or 2 males) or with other females without any males (Fig. 1). In addition, we calculated how often females were expected to be found either alone or in these groups, assuming a random distribution (expected values). The comparison of observed and expected values showed that females avoided staying alone ( 2 4.8, df 1, P 0.05), but they preferred to stay with a single male ( 2 3.9, df 1, P 0.05) or with two males ( 2 142, df 1, P 0.001) when no other females were around. They spent only 5% of the observation time with femaleonly groups ( 2 5.7, df 1, P 0.05). Females showed no pairing with specific males. On average, each female was observed with 1.6 males per observation period. Our study indicates that under more normal conditions female zebrafish might live with one or two males but separated from other females. Males seemed to change female partners on a daily basis. We assume that they were probably attracted by female pheromones indicating female reproductive stage. Bloom and Perlmutter (3) showed that zebrafish, when placed in a T-maze, demonstrated either a preference for or an avoidance of donor water produced when specified numbers of conspecifics of either sex were kept in holding systems for set lengths of time. Their results and ours are consistent with the higher egg production we observed in females kept with one or two males. The conditions under which zebrafish are often kept in laboratories seem to conflict with their natural preferences. Egg production could be increased by considering these observed behavioral patterns.

Influence of Naineris laevigata (Polychaeta: Orbiniidae) on vertical grain size distribution, and dinoflagellate resting stages in the sediment

Short-term experiments were performed with the polychaete Naineris laevigata Grube, a conveyor-belt deposit feeder, to evaluate: the daily sediment reworking rate, the influence of the activity of this burrowing worm on the vertical redistribution of the sediment and dinoflagellate cysts. We also tested the germination success of cysts after their passage through the gut of the polychaete. Vertical particle distribution was studied in small aquaria with and without worms and with an initially layered sediment. After 30 days, sediment cores were collected in controls and treatments at 3 different depths (upper, intermediate and deep) and the diameter of 100 particles from each depth was measured to obtain the size-frequency distribution. The burrowing activity of 3 worms may completely mix a pre-existent layering within 30 days. The translocation of a mud layer from 4 cm depth towards the surface was observed. The mud layer deposited on the surface by the end of the experiment was at least 1 cm thick. Daily average production of faecal pellets was 0.443±0.032 g per worm. From the first day of the experiment, the particle size distribution of the faecal pellets released at the surface revealed the presence of sediment and cysts originating from underlying layers. Tests showed that the germination percentage of the cysts significantly diminished after passage through the guts of the worms. Our results indicate that N. laevigata is an effective vertical mixer of sediment and plays a role in the vertical transport and germination success of resting stages.

Does Acartia clausi (Copepoda: Calanoida) use an area-restricted search foraging strategy to find food?

The swimming behavior of Acartia clausi within small aquaria containing phytoplankton was videotaped, along with unfed controls. From these videos, aspects such as swimming speed, distance traveled, and 2D (horizontal, X, and vertical, Z) headings were then measured from over 75000 X Z positional data in order to quantify their swimming functional response. There was generally no significant difference in swimming behavior between different food levels, although there was a large difference between feeding and unfed controls. Because feeding bouts moved the copepods a short distance at a slow speed, net vertical displacement decreased as feeding-bout frequency increased. Non-feeding individuals showed longer periods of sinking, interspersed with longer-distance, high-speed jumps, which displaced them farther distances than filter-feeding copepods over the same time interval. For both fed and unfed control copepods, a 1D unbiased random walk was adequate to describe their displacement over these very short time periods (10 s). This behavior is consistent with an area-restricted search foraging strategy. The net vertical displacements predicted by combining the experimental data with a 1D random walk model suggest that A. clausi would be retained within even small ∼10 cm phytoplankton patches long enough to fill their gut. However, copepods outside of phytoplankton patches must rely on other means to find patches of food greater than ∼50 cm apart. Potentially, knowledge of a copepod's swimming functional response could enable predictions about the typical spatial and temporal patchiness of its food in situ.

ACOUSTO-FLUIDIC DETECTION OF INSECT LARVAE IN GRAIN

DRT and USDA have entered into a Cooperative Research and Development Agreement (CRADA) to exploit the high sensitivity and signal-to-noise ratio of acousto-fluidic sensors to detect and quantify insects in grain and stored products. ARS has developed a technique for acoustically quantifying insects (including internal feeding larvae) in grain samples. ALFID (Acoustic Location Fingerprinting Insect Detector) detects individual insect sounds (chewing, moving) and cross-correlates them to determine relative time delays between sixteen sensors. This data, arranged in vectors (fingerprints) which indicate sound source locations, is compared and grouped thereby identifying the number of insects. ALFID effectiveness has been limited due to the poor SNR of conventional microphones. Initial experiments with acousto-fluidic sensors, that physically increase the sounds so that they can be detected by low cost electret microphones, have been very promising. SNR improved by 2x and insect detection probability improved by almost 3x. In three separate experiments, more than half the rice weevil larvae (Sitophilus oryzae) detected acousto-fluidically were not discerned conventionally. A 3-stage acousto-fluidic sensor (acoustic gain - 56dB, bandwidth 1 - 7kHz) driving two Tibbetts 251-01 electrets, is able to process sound levels of less than OdB SPL (ref. 0.0002, mbar). Two-week old larvae in a 1 kg grain sample have been detected at distances of over 10cm. Production cost of one sensor will be about $100, including electronics and pneumatic supply (small aquarium pump). Inadvertent export of infested grain (grain visually passed as clean) accounts for multi-million dollar annual losses. It is anticipated that the use of an acousto-fluidic ALFID system should greatly reduce these losses. Monitoring of insect infestations in silos and packaged goods will also be possible. Also, this technology may help advance infestation quantification using large arrays of multiplexed sensors coupled with ARS-developed statistical insect population estimates.

Evolution and discontinuous distribution of Rex3 retrotransposons in fish.

The fish non–long-terminal-repeat (non-LTR) retrotransposon Rex3 has recently been isolated from the platyfish Xiphophorus maculatus (Volff et al. 1999). Complete versions of Rex3 encode a reverse transcriptase (RT) and an apurinic/apyrimidinic endonuclease (fig. 1). Rex3 belongs to the RTE family of non-LTR retrotransposons (Malik and Eickbush 1998; Volff et al. 1999). From all autonomous fish retrotransposons reported to date, Rex3 has the widest distribution observed in teleosts and is present in fish species having diverged 150–200 MYA. We report here a large PCRand Southern blot–based survey of Rex3 evolution including 21 representative teleost species (fig. 1) and 115 Rex3 partial reverse transcriptase sequences (fig. 2). The species chosen include a panel of economically important fishes (salmon Salmo salar, trout Oncorhynchus mykiss, carp Cyprinus carpio, sturgeon Acipenser sturio, mandarin fish Siniperca chuatsi) and several small aquarium teleosts used as models for developmental biology, cancer research and evolutionary studies (zebrafish Danio rerio, medakafish Oryzias latipes, platyfish Xiphophorus maculatus, and other Poeciliidae), as well as the genome project fish, the Japanese pufferfish Fugu rubripes. Rex3 partial RT-encoding sequences could be amplified by PCR from the majority of fish species with at least one of the nine primer combinations tested, but not from the more divergent nonteleost Acipenser sturio (sturgeon) and not from both salmonid species tested (O. mykiss and S. salar) (fig. 1). Using cloned Rex3 probes from X. maculatus and from Anguilla anguilla (European eel) in Southern blot experiments, no specific signal could be detected in A. sturio, O. mykiss, and S. salar even under low-stringency conditions (not shown), confirming the PCR analysis results. According to classical morphological and molecular fish phylogenies (fig. 1; Nelson 1994; Forey et al. 1996; Orti and Meyer 1996), the distribution of Rex3 in teleosts is discontinuous. Hence, Rex3 was lost (or, alternatively, diverged extremely rapidly) in the Oncorhynchus/Salmo lineage after its divergence from the Esox lineage. There are about 1,000 Rex3 copies in the haploid genome of Xiphophorus species (Volff et al. 1999). All other Poeciliidae species included in this study and the related Fundulus displayed a high level of Rex3 reiteration as well (data not shown). Rex3 is present in high copy numbers in the genomes of O. latipes, Oreochrom-

Mechanistic considerations in small fish carcinogenicity testing.

Historically, small fish species have proven useful both as environmental sentinels and as versatile test animals in toxicity and carcinogenicity bioassays. They can be bred in large numbers, have low maintenance and bioassay costs, and have a low background incidence of tumors. However, more mechanistic information is needed to help validate the information garnered from these models and to keep pace with other more fully developed animal models. This paper focuses on mechanistic considerations when using small fish models for carcinogenicity testing. Several small aquarium fish species have proven useful. The Japanese medaka is perhaps the best characterized small fish model for carcinogenicity testing; however, the zebrafish is emerging as an important model because it is well characterized genetically. Both route and methodology of exposure may affect the outcome of the study. Most studies have been conducted by introducing the test compound into the ambient water, but dietary exposures and embryo microinjection have also been used. Other considerations in study design include use of an initiating carcinogen, such as diethlynitrosamine, and differences in xenobiotic metabolism, such as the fact that fish CYP2B is refractory to phenobarbital induction. The small size of these models has perhaps limited some types of mechanistic studies, such as formation and repair of DNA adducts in response to carcinogen exposure. However, improved analytical methods are allowing greater resolution and should be applied to small fish species. Slide-based methods such as immunohistochemistry are an important adjunct to routine histopathology and should be included in study design. However, there is a need for development of more species-specific antibodies for fish research. There is also a need for more fish-specific data on cytokines, serum biochemistry, and oncogenes to strengthen the use of these important test models.

Larval condition and vulnerability to predation: reply to comment by Suthers

Discussion 1538 Scientists recognize that observations and experiments at a variety of spatial and temporal scales and in a variety of different systems are required to obtain a complete understanding of a particular phenomenon. However, scientists also recognize that an individual study can usually only focus on one particular system at a certain time and place and that many different studies are required to achieve a general conclusion. The objective of our study on larval capelin (Mallotus villosus) (Elliott and Leggett 1998) was to provide an experimental test of whether larvae in poor condition had lower relative survival rates than larvae in good condition when exposed to a predator. We employed an experimental design that had been successfully used by a variety of other researchers (Litvak and Leggett 1992; Pepin et al. 1992; Pepin and Shears 1995; Elliott and Leggett 1996) to test selective mortality hypotheses under laboratory conditions in small aquaria. In our abstract, we explicitly stated that the results of our study “suggest that RNA/DNA ratios are not useful indicators of vulnerability to predation for larval capelin.” Clearly, we did not intend to have this conclusion be considered a broad statement that applied to all other systems. As in all scientific disciplines, we presented our data with the intention that other researchers would conduct further studies using a variety of experimental systems and protocols to provide further tests that would either support or refute our results. After a variety of studies on this topic had been conducted, we would hope that a metanalysis would provide a more complete understanding of the effects of larval condition on vulnerability to predation. Ultimately, the results of this research could lead to the incorporation of lar val condition indices into assessments of the future survival probabilities of larvae into fisheries models. Unfortunately, in the comment by Suthers, our conclusions have been overstated and interpreted as a definitive re jection of the utility of RNA/DNA ratios and larval dry weight in the study of larval condition. As stated above, our conclusions pertained to larval capelin and only in the context of the experimental conditions employed. Furthermore, in the second to last paragraph of Elliott and Leggett (1998), we explicitly stated that our conclusions are based on our results, some of which are in contrast with previous studies of larval capelin in small enclosures: “This difference in out come may result from the use of different experimental conditions and predator species or from the possibility that different measures of larval size (length versus dry weight) may not be equivalent in their influence on the vulnerability of larvae to predation.” In other words, variation in the spe cies used or experimental protocol could have caused a different result (as has been reported in many other studies of selective mortality). A general conclusion on the influence

Primary structure and expression of the Xiphophorus DNA-(cytosine-5)-methyltransferase XDNMT-1

Small aquarium fishes become increasingly important in the study of normal vertebrate development and disease. Differential DNA methylation might play a role in these processes. In the teleost Xiphophorus, a well-established animal model for melanoma formation, tumour-specific hypomethylation of the melanoma-inducing gene ONC-X mrk has been observed. We have isolated a cDNA for the DNA-(cytosine-5)-methyltransferase XDNMT-1 from this organism, which encodes the first full-length protein from a fish species. Linkage analysis showed that X dnmt-1 is different from the Xiphophorus tumour suppressor R, which is involved in the transcriptional repression of the ONC-X mrk melanoma oncogene in healthy fish. As methylation has been implicated in the regulation of ONC-X mrk expression, XDNMT-1 might play a role by acting up- or downstream of R. Expression analysis demonstrated that the X dnmt-1 transcript is present in all adult tissues and cell lines tested. However, developing embryos show a spatially and temporally regulated expression pattern suggesting that the enzyme might play a role during development in fish.

COVER-SEEKING BEHAVIOR AND SHELTER USE BY JUVENILE AND ADULT CRAYFISH, PROCAMBARUS CLARKII: POTENTIAL IMPORTANCE IN SPECIES INVASION

Cover-seeking behavior of juvenile and adult crayfish, Procambarus clarkii, was examined in relation to the importance of darkness (shadow) and thigmotactic cues. Crayfish were observed individually in small aquaria 5 times per day, with at least 30 min between observations, for a 3-day period. Their positions were recorded as either within or outside shelter(s) provided. Experiment Ia presented crayfish with a choice between a clear thigmotactic shelter or open area. Adults were observed in the open area significantly more than in the shelter. Juveniles were also observed in the open area more often, but the difference was not statistically significant. Experiment Ib presented crayfish with a choice between a dark thigmotactic shelter and open area. Adults were observed more often in the open area, but the difference was not statistically significant. Juveniles were observed in the dark thigmotactic shelter significantly more often than in the open area. In Experiment II, crayfish were given a simultaneous choice among a clear thigmotactic shelter, a dark thigmotactic shelter, and open area. Adults were observed in both open area and dark thigmotactic shelter significantly more often than in the clear thigmotactic shelter. Juveniles were observed in the dark thigmotactic shelter significantly more often than in the open area and were never observed in the clear shelter. For Experiment III, crayfish were given a simultaneous choice among a dark reduced-thigmotactic shelter, a clear thigmotactic shelter, and open area. Adults chose the dark reduced-thigmotactic shelter significantly more often than the clear thigmotactic shelter, but not significantly more often than the open area. Juveniles chose the dark reduced-thigmotactic shelter significantly more often than either the open area or the clear thigmotactic shelter. Experiment IV gave crayfish a choice among a dark reduced-thigmotactic shelter, a dark thigmotactic shelter, and open area. Adults chose the dark reduced-thigmotactic shelter significantly more often. Juveniles chose the dark thigmotactic shelter significantly more often than the dark reduced-thigmotactic shelter, but not significantly more often than the open area. Darkness appears to be the controlling factor in the cover-seeking behavior of both juvenile and adult P. clarkii. The preferential use of larger shelters by adult P clarkii could reduce the limiting effects of shelter availability and might reduce competition for shelter between adults and juveniles, increasing juvenile recruitment. Differences observed in shelter use between adult crayfishes, P. clarkii and 0. rusticus, which prefer the same shelter as juveniles, may give P. clarkii an advantage over 0. rusticus in a species invasion.